1. Field of the Invention
The present invention relates to magnetic resonance imaging and, more particularly, to a high resolution magnetic resonance imaging system and to components thereof.
2. Description of the Related Art
Magnetic Resonance Imaging (MRI) has proven to be an enormously useful technology both for the detection and diagnosis of human disease as well as for research into the understanding of basic animal physiology. However, current MRI equipment has been limited by achievable signal-to-noise ratio (SNR) and by limitations in the ability to generate homogenous transmit fields for signal excitation, particularly at high magnetic field strengths.
For the acquisition of data from a nuclear magnetic resonance (NMR) signal, four separate components are required. First a static magnetic field must be generated by a permanent magnet generally of the superconducting type. Pursuant to quantum mechanics, the presence of the static magnetic field causes in a subject an energy difference between atomic spins aligned with and against this static magnetic field. The magnitude of the energy difference depends on a variety of factors, including strength of the magnetic field, size of the magnetic moments of individual atomic nuclei, and temperature. In general, a majority of the atomic spins will align with the static magnetic field and a higher energy minority of the atomic spins will align against it. When exposed to an oscillating magnetic field of proper frequency, such as is generated by an alternating current in a radio frequency (RF) coil, some of the lower energy spins aligned with the static magnetic field will be excited to the higher energy state of being aligned against the field. Once the applied transmit RF magnetic field is removed, these excited spins will decay to the lower energy state of alignment with the static magnetic field. During the decay, these spins will generate their own RF magnetic field, which can be electronically detected by the same or a different RF coil and thereby be characterized. In order to determine spatial information about the quantity and properties of the atomic nuclei of the subject, a second set of coils, gradient coils, are used to perturb the static magnetic field. By generating magnetic field gradients, current in this separate set of coils spatially changes the oscillation frequency of the atomic spins by changing the frequency of the nuclear magnetic resonance (NMR) oscillation at appropriate times during transmit and receive, and spatial information regarding the atomic spins can be decoded and converted into an image. The generation and reception of the NMR signal in the RF coil and the currents in the gradient coils are controlled by a computer system which processes the information obtained and displays it on a computer screen or printed film for human interpretation.
The advantages of using NMR are several-fold. First, information can be obtained non-invasively on a wide variety of in vitro and in vivo subjects. The lack of non-ionizing radiation is particularly attractive when images are obtained from human subjects. Second, the properties of the magnetic spins are extremely sensitive to their surrounding chemical environment. This allows a great deal of information to be determined from the magnetic resonance signal, including chemical and molecular structure of a wide variety of materials as well as the chemical and structural characteristics of animal and human tissue. By obtaining spatially dependent information regarding the NMR signal, it is possible to obtain detailed images, which not only show great anatomic detail, but which also depend on the chemical properties of tissue. This provides additional image contrast, allows improved discrimination between healthy and diseased tissue, and permits researchers to obtain previously unavailable information regarding in vivo physiologic function.
Despite the multiple advantages of MRI, one major limiting factor in the usefulness of the NMR machine is the small magnitude of the NMR signal generated by a subject's nuclei themselves. This weak signal is easily obscured by the noise present in all electronic detection devices. The presence of this noise then limits the maximum achievable resolution or sensitivity of the NMR machine, specifically, its ability to resolve small anatomic details or to characterize time dependent changes in signal intensity, which are important for understanding of a subject's physiology.
In principle, one can improve the sensitivity of the NMR device by increasing the strength of the static magnetic field. While this does increase signal to noise ratio (SNR), it adds problems in terms of the interaction of high frequency magnetic fields and human tissue, leading to difficulties in achieving uniform image quality and even excitation of NMR spins. Simply increasing the magnetic field strength is a very expensive option: a 3T (3-Tesla) human size magnet costs roughly five times that of a 1.5T magnet. In general, such increased cost places a premium on maximizing SNR at a given field strength.
Most of the noise in human MRI comes from the resistance associated with conductive tissue within the human body. As this resistance is roughly proportional to volume of tissue, large coils, which couple to larger volume of tissue, inherently produce lower quality images than smaller coils. While sensitivity can be improved by making smaller coils, there is a limit to this approach in that eventually the desired body part or region of interest will not fit within the coil or field of view of the coil.
One prior art method designed to increase the field of view of small coils is to use multiple coils arranged in a “phased array” (U.S. Pat. No. 4,887,039). In this method, the images from each individual coil are processed separately and then combined in such a fashion as to maximize image quality. While this is a useful strategy, it has certain limitations. First the individual coils need to be carefully oriented to minimize their respective coupling. Despite proper orientation, there always will be residual coupling between four or more coils limiting the maximum number of coils and consequently the gains in sensitivity. Furthermore, in the standard geometry feasible with surface coils, this arrangement still produces inhomogeneous images, which can complicate their interpretation for diagnostic purposes.
A second problem is the efficient and uniform excitation of the NMR spins. For most imaging sequences, a homogenous excitation of all spins is required. In general, this requires a larger coil, which then reduces the sensitivity of the system. One commonly used technique is to use a larger coil, optimized for transmit with a second coil specialized for receive. However such systems, as presently implemented, suffer from several disadvantages, particularly when used in high field systems. One disadvantage of current volume transmit coils is the inability to control the field to compensate for variations in patient size and position. While, in principle these variations can be accomplished by manually tuning the coil (see J. Thomas Vaughan, Hoby P. Hetherington, Joe O. Out, Jullie W. Pan, Gerald M. Pohost, “High Frequency Volume Coils for Clinical NMR Imaging and Spectroscopy”, Magnetic Resonance in Medicine 32:206–218 (1994)) or by using electromechanical relays to switch in additional reactive circuit elements, such methods are time consuming and subject to the variability of mechanical connections.
Conventional MRI coils come in two basic categories. (1) The simpler, the surface coil, consists of one or more conductive loops. Additional reactive circuit components, such as capacitors and inductors, are used to tune the coil and couple energy to or from it to the rest of the NMR system. Importantly, active circuit elements, such as PIN diodes, can be added to allow specialization of coil function for receive or transmit. (2) Volume coils, such as birdcage coils, consist of one or more large surface coils oriented in such a fashion as to produce a homogenous magnetic field. While such coils are in common use, the large size of these coils makes them poor receivers of NMR signal. This difficulty can be overcome by using PIN diodes to “detune” the volume coil for use with a more sensitive surface coil receiver.
In particular, at high fields, the use of volume coils becomes increasing problematic. The large size of these coils required to enclose a useful area of human anatomy, such as the torso or head, leads to them becoming efficient radiators of electromagnetic energy. Moreover, the interaction of large volume coils with tissue at high frequencies leads to non-uniform magnetic fields within human tissue complicating the ability to obtain uniform spin excitation.
The following U.S. Pat. No. 5,557,247 to Vaughn, U.S. Pat. No. 4,751,464 to Bridges, U.S. Pat. No. 4,746,866 to Roschmann and U.S. Pat. No. 4,506,224 to Krause, disclose volume coils based on cavity resonators. Conductive segments within the cavity interact to form a resonant structure. While this coil can offer improved efficiency over a conventional volume coil, several disadvantages exist. First, the structure being closed can give a subject a sense of claustrophobia and make it difficult to present visual stimulation for research purposes. Second, the closed shielded nature of the coil makes it difficult to specialize for the use of transmit or receive purposes. If circuit elements are added to detune the coil, the outer cavity shield will interact with smaller coils placed with the larger cavity, impairing their performance. Additionally, the cavity shield prevents the use of the coil for specialization as a smaller coil to use with receive only function or as its use as a phased array.